In fuel cell systems generating power by circulating a hydrogen gas, which is a fuel gas, in a fuel cell, an air supply pressure is controlled according to a power generation quantity of the fuel cell, that is, to load fluctuations, and the amount of consumed hydrogen gas is increased or decreased by changing the rotation speed of a hydrogen pump that causes forced circulation of the hydrogen gas.
For example, Japanese Patent Application Laid-open No. 2003-68334 discloses a method by which the air pressurized in a compressor is introduced into a fuel gas pressure regulating valve and the difference in pressure between the poles of a fuel cell is regulated to be maintained within the prescribed range. The fuel gas is supplied with a hydrogen pump controlled so that the revolution speed thereof increases according to the required output of the fuel cell. A method by which a fuel gas is similarly supplied with a turbine is disclosed in Japanese Patent Application Laid-open No. 60-10566.
However, in the above-described well-known methods, the regulation between the drive quantity of the drive means such as a hydrogen pump or a turbine and the pressure regulation quantity of the circulation route is not considered, and the possibility of adverse effects is especially high when the fuel supply quantity is varied only with the drive means.
Thus, in the drive means such as a pump, because the power consumption of the power source increases with the increase of fuel cell load, the total power generation efficiency of the system tends to degrade when the load is high.
Furthermore, if the fuel supply in a high-load mode of a fuel cell is considered to depend only on the circulation quantity control provided by the drive means, the drive means is required to withstand high-speed rotation and circulate a large amount of fuel gas. For this reason, the drive means has to be increased in size.